1. Field of the Invention
This invention relates to a front-end amplification circuit, and more particularly to a preamplifier of the transimpedance type for an optical receiver for use with an optical transmission system.
2. Description of the Related Art
Several examples of a conventional transimpedance type preamplifier of the type described above are disclosed in Japanese Patent Laid-Open Nos. 89632/1989, 82804/1990, 278906/1990 and so forth. An exemplary circuit construction of a conventional preamplifier is shown in FIG. 1. Referring to FIG. 1, an optical signal is converted into photo-current Iph by light receiving element Dph, and photo-current Iph flows through feedback resistance element Rf connected to the base of input stage emitter-grounded transistor T81, which composes a front-end amplification circuit, into the front-end amplification circuit, by which it is converted into a voltage. The conversion voltage is extracted as output OUT from output stage emitter follower transistor T83.
The collector output of emitter-grounded transistor T81 in the input stage is supplied as a base input to collector-grounded transistor T82, whose emitter output is supplied as a base input to output stage emitter follower transistor T83. Here, diode D83 to the emitter of transistor T82 has a level shifting function. Reference symbol R81 denotes a collector resistor to transistor T81, R82 an emitter resistor to transistor T82, and R83 an emitter resistor to transistor T83.
In the front-end amplification circuit shown in FIG. 1, as photo-current Iph increases, the voltage drop by feedback resistance element Rf increases, and the potential at a junction between diode D83 and feedback resistance element Rf approaches low power supply potential VEE. In this instance, collector current Ic1 of transistor T81 operates in an increasing direction. If collector current Ic1 increases until collector-emitter voltage VCE of transistor T81 is saturated by the voltage drop by resistor R81, then the entire circuit does not operate regularly.
As a method for preventing the saturating operation, a method is available wherein, as shown in FIG. 2, diode D84 for limiting the voltage drop is connected in parallel to feedback resistance element Rf. It is to be noted that, in FIG. 2, like elements to those of FIG. 1 are denoted by like reference symbols, and the circuit shown in FIG. 2 is same as the circuit of FIG. 1 except that it additionally includes diode D84 mentioned above.
In the circuit of FIG. 2, when the relationship between photo-current Iph and the voltage drop of feedback resistance element Rf comes to satisfy the following expression: EQU Rf.times.Iph&gt;Vf(D84)
forward voltage Vf(D84) of diode D84 becomes dominant, and the dropping voltage is limited by this forward voltage Vf(D84).
However, such a circuit construction as described above has the following drawback. In particular, since the voltage drop when the input power is high becomes fixed, from a point of view of circuit operation, it is considered that feedback resistance value Rf becomes lower equivalently. This is likely to give rise to unstable operation of the feedback circuit.
In FIGS. 3A to 3C, a frequency characteristic (transimpedance characteristic) (FIG. 3A) of this circuit, an open loop gain band characteristic (FIG. 3B), and a no-feedback gain band characteristic (FIG. 3C) of a core amplification section (transistors T81, T82). In FIG. 3, reference symbol A denotes an open loop gain in a low frequency band, and Cin denotes an input capacitance value in the input stage of the core amplification section.
As seen from FIG. 3C, a gain margin at a frequency of a primary pole of a no-feedback gain is an index indicating a degree of stability of the feedback circuit. However, when feedback resistance element Rf decreases equivalently, the zero-cross point of the open loop gain shifts to the higher frequency band side in accordance with A/2.pi..multidot.Cin.multidot.Rf) and approaches the primary pole of the no-feedback gain band. Consequently, the gain margin is reduced to zero and a condition in which oscillation is very likely to occur is entered.
Further, since light receiving element Dph is connected to the base of input stage transistor T81, a capacitance component originating from light receiving element Dph is added. Consequently, the circuit construction of FIG. 2 is disadvantageous in that the addition of the capacitance component has a bad influence on the band characteristic and the noise characteristic of the front-end amplification circuit.